In French patent No. 1 351 572, the Applicant has already proposed an oscillating mechanical device comprising a shaft having an oscillating member mounted thereon and which is urged resiliently towards an angular equilibrium position by means, in particular, of a torsion bar housed inside the shaft. The shaft is coupled to the rotor of a single-phase induction motor. In theory, once the oscillating device has been set into motion, the shaft oscillates with oscillations of given amplitude and at a fixed frequency which is equal to the natural frequency of the mechanical device and which is independent of the frequency of the AC drive signal fed to the stator of the motor. This natural frequency depends in a known manner solely on the torsion characteristics of the resilient return means and on the moment of inertia of the masses subjected to rotary drive, i.e., in particular, the inertia of the shaft, of the member carried by the shaft, and of the rotor. Once the device is oscillating at its natural frequency, the single-phase induction motor serves solely to provide sufficient energy to compensate for damping due to friction, thereby sustaining oscillations.
Such an oscillating device is used more particularly in the textile industry as a simple way of implementing a card web comb for detaching the web at the outlet of a carding machine, and at present the highest possible rate of oscillation is less than 4,000 strokes/minute, which corresponds to a natural frequency of oscillation of about 66.7 Hz. In practice, e.g. with combs oscillating at 2,817 strokes/minute, i.e. combs whose natural frequency of oscillation is 46.95 Hz, the frequency of the AC drive signal fed to the stator of the single-phase induction motor is adjusted to between 53 Hz and 63 Hz. It is known that in this frequency range, the amplitude of comb oscillations is stable and reaches a maximum at a frequency of 54 Hz. Also, in compliance with theory concerning the operation of a card web comb as recalled above, within the range of frequencies and whatever the frequency of the signal fed to the stator, the frequency of oscillation of the comb is constant and equal to the natural frequency of the comb. The oscillations of the comb are thus completely asynchronous relative to the driving frequency applied to the stator of the single-phase induction motor.
A single-phase induction motor is a motor of simple design and it is reliable. When such a motor is used to implement card web combs, it serves advantageously to reduce maintenance costs, compared with using motors that are more sophisticated, e.g. brushless motors.
However, until now, the use of a single-phase induction motor for sustaining oscillations of an oscillating device of the card web comb type has suffered from two main drawbacks.
The first drawback is associated with the fact that a single-phase induction motor draws a large amount of current since its rotor is subjected to frequent reversals in its direction of rotation under the effect of the resilient return action applied to the shaft which is coupled to the rotor. This drawback leads to using an induction motor rated for power that is overdimensioned compared with the rate at which it needs to feed mechanical energy to the comb in order to sustain oscillations.
The second drawback is associated with starting a card web comb that uses a single-phase induction motor. On starting, when the rotor of the single-phase induction motor is stationary and voltage is applied to its stator, the motor develops zero driving torque, so it is necessary to impart a preferred direction of rotation to the rotor in order to start it. When stationary, the single-phase induction motor behaves like a single-phase transformer whose primary and secondary windings carry very high currents. The rotor, and consequently the torsion bar, then begins to vibrate under the effect of intense eddy currents which are induced in the magnetic masses of the motor. In theory, these vibrations serve automatically to start oscillations in the shaft at the natural frequency of oscillation of the mechanical device, and the angular amplitude of the oscillations tends to increase rapidly until it reaches a stable maximum value. In practice, oscillations do not always start. Therefore, to trigger oscillations it is necessary either to start the comb manually by applying torque to the torsion bar, or else to equip the single-phase induction motor with an external starter device.
In order to mitigate the above-specified problems, attempts have been made to replace the single-phase induction motor with a motor that is more sophisticated, e.g. of the brushless motor or DC motor type. That is the solution recommended in European patent EP 519 878 which, in general terms, teaches firstly replacing the single-phase induction motor with a motor that delivers constant torque regardless of the speed of rotation of the rotor, and whose direction of rotation can be reversed under electrical control, and secondly servo-controlling reversal of the direction of rotation as a function of the amplitude of comb oscillation so as to provide an oscillating system. The same solution is adopted in Russian patent application SU 1 227 726.